The quantal response to a single photon is the elemental event in vision. In rods, this discrete electrical event results from activation of an enzyme cascade that closes hundreds of channels in the plasma membrane. Absorption of a photon normally initiates the process of photoexcitation by isomerization of the chromophore retinal from 11-cis to the all-trans configuration within the visual pigment, rhodopsin. After excitation, all- trans retinal is released from the apoprotein opsin, and ultimately replaced with fresh 11-cis retinal supplied by the pigment epithelium. The size and shape of the quantal response are governed by l) the intramolecular events which result in formation of Metarhodopsin II and the exposure of a catalytic binding site for transducin, a GTP-binding protein, 2) deactivation of Metarhodopsin II by phosphorylation of the carboxy terminus and capping by arrestin, 3) nearby molecules of photoactivated rhodopsin that produce background adaptation, 4) nearby molecules of opsin that produce opsin desensitization and 5) the availability of 11-cis retinal to convert opsin to its inactive rhodopsin configuration. The longterm objective of this investigation is to determine the mechanism(s) of retinoid uptake into photoreceptors and to gain a molecular explanation of the influence of the retinoids in the mechanisms of excitation and adaptation. As outlined in the progress report, work initiated in this laboratory has demonstrated that it is possible to manipulate excitation, adaptation and retinoid uptake with analogues of retinal. The Specific Aims of this proposal are to exploit these observations with an analysis of the physiological activities of selected analogues of retinal in intact cells. To this end, we propose: l) to determine the minimum structural requirements of retinal for the relief of opsin desensitization and for the control of excitation in rods & cones, 2) to identify the biochemical pathway responsible for opsin desensitization. 3) to determine the mechanism of retinoid uptake in rods and cones. The proposed work is addressed to program priorities of the NEI panel on Retinal and Choroidal Diseases. The work addresses fundamental mechanisms of receptor activation and desensitization with health ramifications relating to the efficacy of drugs and to the anomalies of pigment activation and dark adaptation associated with retinitis pigmentosa.